This report details modelling work conducted to develop a comparison of Hatfield and alternative rail steels through examination of the effect of decarburisation on crack initiation. The input data on decarburisation comes from both the experimental data collected in project JR31.086, and alternative data available from published literature. Also included in this report is validation work on the hybrid two and three dimensional fracture mechanics based crack growth model ("2.5d" model) used in project JR31.086 to predict the effect of residual stress on crack growth. This validation has been conducted using newly developed fully three dimensional models, but could not be based on Hatfield specific rail residual stress data because this was not available. Use of alternative input data does not affect this validation, since examination of agreement between models is independent of the residual stress profiles used, providing that the same profiles are used in both modelling approaches.